Liquid ink printers of the type frequently referred to as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal, have at least one printhead from which droplets of ink are directed towards a recording medium. Within the printhead, the ink is contained in a plurality of channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
In a thermal ink-jet printer, the power pulses are usually produced by resistors each located in each one of the respective channels and individually addressable by current pulses to heat and vaporize ink in the channels. A thermal energy generator, usually a resistor or a heater, is located in each of the channels, a predetermined distance from the nozzles. The resistors are electrically individually addressed with a current pulse to momentarily vaporize the ink thereby forming a bubble which expels an ink droplet. As the bubble grows, the ink which bulges from the nozzles, is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink remaining in the channel between the nozzle and the bubble move towards the collapsing bubble causing a volumetric contraction of the ink at the nozzle resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards the recording medium. The droplet of ink lands on the recording medium and forms an ink spot. Because the droplet of ink is emitted only when the resistor is actuated, this type of ink jet printing is known as drop-on-demand printing. The channel is then refilled with ink by capillary action, which, in turn, draws ink from a supply container. Operation of a thermal ink-jet printer is described in, for example, U.S. Pat. No. 4,849,774.
One particular form of ink jet printer is described in U.S. Pat. No. 4,638,337. The described printer is of the carriage type and has a plurality of printheads each having its own supply cartridge mounted on a reciprocating carriage. The nozzles in each printhead are aligned perpendicularly to the line of movement of the carriage and a swath of information is printed on the stationary recording medium as the carriage is moved in one direction. The recording medium is then stepped perpendicularly to the line of carriage movement by a distance equal to the width of the printed swath. The carriage is then moved in the reverse direction to print another swath of information. Full width or page width linear arrays in which the sheet is moved past a linear array of nozzles extending across the full width of the sheet, are also known.
In a typical ink-jet printing machine, the carriage must transport the printhead assembly across the page for printing and must also move the carriage to predetermined locations for capping, priming, and other maintenance functions for the printhead and the printhead nozzles thereof. In each of these instances, the carriage is moved across the recording medium in a controlled fashion or is parked at the predetermined locations along the carriage rails. A carriage motor and electronic controller are provided to precisely position the carriage at these locations. Since a motor is typically used, the rotary motion of the motor, is converted to the linear motion of the carriage by among others, a toothed belt/pulley, a cable/capstan or a lead screw. In addition to these devices, which move the carriage in a linear fashion, the linear motion is controlled and/or kept track of by an encoder.
Linear and rotary encoders are used for positioning and timing of movable members. In linear encoders, a linear strip of material includes a plurality of markings called fiducial markings, which are typically illuminated by a source of light and detected by an optical sensor to determine positioning and timing. The optical sensor detects the fiducial markings and generates a series of electrical pulses which are transmitted to a control system for controlling the motion of a movable member, such as a printhead carriage. The linear strip of fiducial markings is mounted on the printer is parallel to the anticipated path of the carriage as it traverses across the recording medium. The light source and sensor are mounted on the carriage so that as the carriage reciprocates back and forth across the recording medium the combination light source/sensor can illuminate and detect the fiducial markings on the encoder strip for controlling the motion of the printhead carriage.
Rotary encoders use a disk coupled to a rotating member in which the disk includes a plurality of spaced marks. The marks are arranged on the disk so that as the marks rotate with the rotating member an illumination source/sensor senses the marks for determining the position, velocity and acceleration the rotating member. The illuminating source and the sensor can be disposed on opposite sides of the rotating disk to sense the passage of marks if the disk is transparent to light. In this way, a pulse is generated for each increment between adjacent marks of the disk.
In both the linear strip and disk encoders, the fiducial markings are typically spaced a predetermined distance apart related to a printing resolution for controlling the motion of the moving member. These fiducial marks are typically produced via a photographic or etching process. Once the strip or disk has been made, the encoder strip or disk is mounted on a support member such as a stationary platform, as in the case of monitoring the position of a printhead carriage, or a moving platform when the disk is mounted on the rotating member. Because it is desirable to accurately control the motion and/or position of the moving member, accurate placement of the encoder strip or disk is critical. Consequently, the encoder strip or disk must be positioned accurately on the support member or the member which is to be controlled. Typically, the positioning of the strip or disk must be made to a fairly tight tolerance to assure accurate control of the moving member.
Various printers and methods are illustrated and described in the following disclosures which may be relevant to certain aspects of the present invention.
In U.S. Pat. No. 5,394,223 to Hart et al., an apparatus for image registration is described. The apparatus positionally tracks a moving photo conductive belt and adjusts an imager in an electrophotographic printing machine to correct for alignment errors when forming a composite image. Registration errors are sensed by developing an appropriate set of target marks, detecting the target marks and controlling the position of the imager.